Chapter 1 Part B: Structure and Bonding, acids and bases
Summary
TLDRThis educational script delves into the nature of polar covalent bonds, emphasizing the unequal electron distribution between atoms of differing electronegativity. It contrasts polar covalent bonds with nonpolar and ionic bonds, using examples like carbon-oxygen and carbon-lithium bonds. The script also covers the concept of electronegativity, highlighting fluorine as the most electronegative element and the trend's increase from cesium. It explores inductive effects, acid-base reactions with Bronsted-Lowry theory, and the significance of pKa values in determining acid strength. The discussion extends to organic acids and bases, including their structures and roles in reactions, and touches on Lewis acids and bases. Lastly, it addresses the practical aspects of organic chemistry in food production, discussing the use of pesticides like atrazine, their benefits, and the risk-benefit analysis in agriculture.
Takeaways
- đŹ Polar covalent bonds occur when there's an unequal sharing of electrons due to a difference in electronegativity between the bonded atoms.
- đ The electronegativity scale ranges from cesium (0.7) being the least electronegative to fluorine being the most electronegative.
- đ Nonpolar covalent bonds are formed between atoms of similar electronegativity, while polar covalent bonds form between atoms with significant electronegativity differences.
- đ The inductive effect describes how the electron distribution in a molecule is influenced by the electronegativity of nearby atoms.
- đ§Ș Bronsted-Lowry acid-base theory defines acids as proton (H+) donors and bases as proton acceptors, with pKa values indicating acid strength.
- đ A lower pKa value corresponds to a stronger acid, as it indicates a greater tendency of the acid to donate a proton.
- đ¶ Organic acids often contain polarized hydrogen atoms that can be lost, typically found next to oxygen or a carbonyl group.
- đ§Ș Organic bases are compounds with atoms that have lone pairs of electrons available to bond with H+ ions, often nitrogen-containing.
- đŹ Lewis acids and bases differ from Bronsted-Lowry definitions, focusing on electron pair acceptance and donation, respectively.
- đ± The term 'organic' in food does not necessarily imply a lack of synthetic chemicals or pesticides; it's a label that can be misleading without regulation.
Q & A
What is a polar covalent bond?
-A polar covalent bond is a type of covalent bond where the electron distribution is not equal due to a difference in electronegativity between the two bonded atoms. The more electronegative atom attracts the bonding electrons more strongly, creating a dipole.
How does electronegativity affect the polarity of a bond?
-Electronegativity is the intrinsic ability of an atom to attract electrons. The difference in electronegativity between two atoms in a bond produces bond polarity, with the more electronegative atom pulling the electron density towards itself.
What is the difference between a nonpolar covalent bond and a polar covalent bond?
-In a nonpolar covalent bond, the electronegativities of the two atoms are similar, resulting in an even distribution of electron density. In contrast, a polar covalent bond has a difference in electronegativity, leading to an uneven distribution of electron density and the formation of a dipole.
What is the most electronegative element?
-Fluorine is the most electronegative element, with the highest tendency to attract electrons in a chemical bond.
How does the inductive effect influence the polarity of a molecule?
-The inductive effect is the shifting of electrons within a molecule in response to the electronegativity of nearby atoms. This can alter the distribution of electron density, affecting the overall polarity of the molecule.
What is the difference between a Bronsted acid and a Bronsted base?
-A Bronsted acid is a substance that donates a hydrogen cation (proton), while a Bronsted base is one that accepts a hydrogen cation. The reaction between them results in the formation of a conjugate acid and a conjugate base.
What is the significance of pKa values in acid-base chemistry?
-pKa values, which are the negative logarithms of the acid dissociation constant (Ka), measure the strength of an acid. A smaller pKa value indicates a stronger acid, as it means the acid is more likely to donate a proton.
How can you predict the direction of an acid-base reaction using pKa values?
-You can predict the direction of an acid-base reaction by comparing the pKa values of the acids involved. The reaction will favor the formation of the conjugate base from the weaker acid, as it is less likely to accept a proton.
What is a Lewis acid and how does it differ from a Bronsted acid?
-A Lewis acid is an electron pair acceptor, which can accept electrons directly from a Lewis base, an electron pair donor. Unlike Bronsted acids, which donate protons, Lewis acids do not necessarily involve proton transfer.
Why might the FDA allow the use of certain pesticides like atrazine despite potential health risks?
-The FDA may allow the use of certain pesticides, such as atrazine, because the benefits of increased crop yield and reduced food costs can outweigh the potential health risks. The decision is often based on a risk-benefit analysis that considers both short-term and long-term exposure risks.
Outlines
đŹ Understanding Polar Covalent Bonds
This paragraph delves into the nature of polar covalent bonds, explaining how they form when there is an unequal attraction of bonding electrons by the atoms involved. It contrasts this with nonpolar covalent bonds, where the electron distribution is even due to similar electronegativities of the atoms. The concept of electronegativity is introduced as the intrinsic ability of an atom to attract electrons, with fluorine being the most electronegative element. The paragraph also discusses how differences in electronegativity can lead to bond polarity and eventually ionic bonds if the difference is significant. Examples of polar covalent bonds are given, such as carbon to oxygen or oxygen to hydrogen. The inductive effect, which is the shifting of electron density in response to nearby atoms' electronegativity, is also mentioned, along with a brief explanation of Bronsted-Lowry acid-base concepts.
đĄ Acid-Base Reactions and pKa Values
The second paragraph focuses on acid-base chemistry, specifically the Bronsted-Lowry definition where acids donate protons (H+) and bases accept them. It explains the concept of conjugate acids and bases using the example of HCl reacting with water. The paragraph introduces pKa and pKb values, which are measures of acid and base strength, respectively, with lower values indicating stronger acids or bases. The pKa of water is given as a reference point, and the paragraph explains how these values can be used to predict the direction of acid-base reactions. It also discusses the relative strengths of different acids and bases, including organic compounds like ethanol and HCl, and how their pKa values relate to their strength.
đ Organic Chemistry in Food Production
This paragraph explores the application of organic chemistry in food production, addressing misconceptions about what 'organic' means in the context of food. It clarifies that organic farming still involves the use of chemicals and pesticides, albeit often natural ones, and that the FDA does not strictly regulate the term 'organic.' The discussion then turns to the use of pesticides like atrazine, which is widely used to control weeds in crops but can have environmental and health impacts. The paragraph presents a risk-benefit analysis, comparing atrazine's benefits in increasing crop yields with its potential risks, and explains why it is still allowed for use despite these concerns. It also touches on the use of other chemicals in farming, such as herbicides, insecticides, and fungicides, and the challenges of maintaining crop production without them.
Mindmap
Keywords
đĄPolar Covalent Bonds
đĄElectronegativity
đĄDipole Arrow
đĄNonpolar Covalent Bonds
đĄIonic Bonds
đĄInductive Effect
đĄBronsted-Lowry Acids and Bases
đĄpKa
đĄLewis Acids and Bases
đĄOrganic Chemistry in Food
Highlights
Polar covalent bonds form when there's an unequal attraction of bonding electrons by the bonded atoms.
Covalent bonds between two identical atoms have equal electron distribution, resulting in nonpolar bonds.
Electronegativity differences between atoms in a bond lead to polar covalent bonds.
Electronegativity is the ability of an atom to attract electrons, with fluorine being the most electronegative element.
Nonpolar covalent bonds occur between atoms with similar electronegativity, such as carbon and hydrogen.
Polar covalent bonds are exemplified by carbon to oxygen, oxygen to hydrogen, and carbon to halogen bonds.
Ionic bonds typically form between metals and nonmetals, with the electronegative atom gaining all electron density.
Methanol and methyl lithium illustrate the difference between polar covalent and ionic bonds through electron density maps.
The inductive effect describes the shift of electrons in a bond due to the electronegativity of nearby atoms.
Ethanol is a polar compound due to the uneven distribution of electron density, as shown by its molecular structure.
CCL4 has polar bonds but is a nonpolar molecule because the polarities of its bonds cancel each other out.
Bronsted-Lowry acid-base theory defines acids as proton donors and bases as proton acceptors.
pKa values, the negative logarithm of the acid dissociation constant (Ka), measure the strength of acids.
A smaller pKa indicates a stronger acid, with water's pKa being 15.74, making it a weak acid.
Organic acids are characterized by the presence of polarized hydrogen atoms that can be lost as protons.
Organic bases have atoms with lone pairs of electrons that can bond to H+, with nitrogen-containing compounds being the most common.
Lewis acids accept electron pairs and are different from Bronsted acids, which do not accept electrons directly.
Most oxygen and nitrogen compounds are Lewis bases due to their lone pairs of electrons.
The food industry's use of the term 'organic' can be misleading as it does not necessarily mean absence of synthetic chemicals or pesticides.
Atrazine, a widely used herbicide, is an example of a chemical allowed in organic farming despite potential health risks.
Risk-benefit analysis is used to justify the continued use of atrazine in agriculture due to its benefits outweighing the risks.
Transcripts
polar covalent bonds are bonds that are
present when the bonding electrons are
attracted
by one atom than by the other so it just
means that the electron distribution is
not the same if we look at a covalent
bond between two of the same atoms
they're going to have similar polarities
if you have one that has this slightly
more electronegative
then the way that your electron density
is going to be weighted is slightly more
on the electronegative atom versus this
one over here so if you have a
difference of electronegativity between
two atoms in a bond the one that is more
electronegative is more likely to carry
more of the electron density than the
other so this would be a covalent bond
where it's shared equally this is a
polar covalent bond where you can draw a
dipole arrow if there's enough of an
electron difference it becomes an ionic
bond where the electronegative atom is
carrying all of the electron density and
this is taking none of that
so the electronegativity is the
intrinsic ability of an atom to attract
electrons the differences in
electronegativity between two atoms in a
bond produces bond polarity or fluorine
is the most electronegative one and the
trend increases as you move in this
direction so the lowest electronegative
atom is cesium at 0.7 and halogens and
nitrogen oxygen tend to be more
electronegative so with non covalent
bonds you have atoms with similar
electronegativity
en is abbreviation for electronegativity
CH is an example of two atoms that have
nonpolar covalent bonds polar covalent
bonds have a difference in
electronegativity of atoms
so the examples of these would be carbon
to oxygen oxygen to hydrogen or carbon
to a halogen ionic bonds generally occur
between a metal and a nonmetal
so if we look at these two examples here
we have methanol which is a carbon bond
between carbon and oxygen and that
electron density you can see her from
this electron density map the red
indicates that it's negative this is
going to be positive so we have the
dipole arrow here with the electron
density on the oxygen and the partial
positive on the carbon with methyl
lithium this is more of an ionic bond
between carbon and lithium you can see
this is very blue versus this was a
slight green color and this is very red
so the dipole arrow is drawn in the
opposite direction where carbon now has
that negative charge lithium is positive
what an inductive effect is is a
shifting of electrons in a bond in
response to the electronegativity of
nearby atoms which we can see with this
carbon in the methyl lithium
you
and we'll see more examples of inductive
effects later on alright so for our
practice problem use the partial
positive partial negative to indicate
the direction of expected polarity for
each of these so we have ch3 ch2 O H if
you were to draw those out I have ch3
ch2 and then Oh H oxygen has two lone
pairs each carbon has four bonds oxygen
has two like it should if I look at the
electron density of this one the carbon
is partial positive because it is less
electronegative than oxygen which is one
of the electronegative atoms the dipole
arrow was going to move in this
direction so this is the structure of
ethanol so that is a polar compound
because there's an uneven distribution
of electron density if we look at CCL 4
we have C CL CL CL CL chlorine is very
electronegative because it's right under
fluorine so I can draw partial positive
here partial negative to each of these
chlorines
and dipole arrows to each of these as
well
so there are polar bonds here
but because each of these are pulled in
different and equal directions the
overall molecule is nonpolar
if we look at the definition of a
bronsted-lowry acid base which is often
shortened to bronsted a bronsted acid is
a substance that donates a hydrogen
cation
and the bronsted base
one that accepts the H+
H+ can also be referred to as a proton
a proton is a synonym for H+ if we look
at this reaction here we have HCl plus
HOH which gives CL minus and h3o plus if
we try to figure out what is acid and
what its base we can look at the other
half of the reaction so if we notice
here the h and the CL on the other side
this is losing the h which means that
this HCl is the one that donated it the
one that donates is the acid
which by default makes this the base to
figure out the conjugate acid and
conjugate base we can look at the
reaction going backwards in this case
the CL will CL minus and it becomes HCl
so it is accepting a proton which means
this is the conjugate base
and by default this is the conjugate
acid and the conjugate acid is the
is going to be donating a proton h3o
plus donates its proton to CL - which
makes it h2o we can use PKS
which is the negative log of ka to
measure the acid strength PKA is the
negative log of ka this measures acid
strength
so if you have a smaller PKA this
indicates a stronger acid
the pKa of water is fifteen point seven
four this is a values you should
definitely become familiar with so what
a ka or PKA tells us is how far the
reaction proceeds in an acid-base
reaction so a KA is a concentration of
acid times base over H a which is also
the same as h3o plus times o h- over
water and you can see these values here
so for water it is 1.8 times 10 to the
negative 16 the pKa equals fifteen point
seven for the KA is a measure of how
much of the acid will dissociate into
hydronium ion and conjugate base we can
look at their relative strengths of
different common acids ethanol is a
weaker acid water as we know is a weak
acid and we forget down to HCl that is a
very strong acid if you notice the pKa
is of ethanol 16 negative seven is a
very strong acid for HCl and the
conjugate base of each of these you can
observe here as well so a weaker acid
stronger acid weaker base over here
stronger base so one thing that you
might observe is a stronger acid
means that it will have a weaker
conjugate base
so this is HCl which is a strong acid
but cl- is a weak base you will not need
to memorize these PK's except for water
so if we want to rank these substances
in order of creasing acidity we want to
remember that high PKA
hi P ka equals weak acid so the value
with the highest will be the least
acidic
and the one with the lowest value will
be the most acidic so if I put this in
order acetone is a pKa of 19 the pentane
Dino diode has a pKa of 9 this is nine
point nine four point seven six the
weakest one will be acetone the
strongest is four point seven six
then 9.9 the phenol is next
and then the pentane died on
so we can just move these from highest
pKa to lowest PKA
this is least acidic
this is most acidic
we can use PKS to predict how the
reaction is going to proceed if we have
acetic acid with hydroxide ion acetate
ion plus water we can use the pKa values
that are related to their logarithms to
determine the equilibrium constant so we
can use this to determine whether or not
a reaction will happen which side the
equilibrium lies on so if you have a
higher PKA that means it's harder to
remove a proton
proton being H+ so we just saw in the
last slide the pKa of acetic acid was
around 4.8 water is fifteen point seven
four so when you're trying to predict
the acid-base reactions compare acids
because we have the PKA values to
support that the value with the higher
PKA is water which has fifteen point
seven four which means it's going to be
harder to remove this proton if it's
hard to remove this proton that means
that most of the product is going to be
on this side because the acetate ion
will not be able to take that proton off
to go to this side so this side of the
equilibrium is favored
so the equilibrium favors the high PKA
because the high PKA means that we can't
remove this proton very well if we can't
take the proton off then it can't go in
the opposite direction
organic acids come in different shapes
and sizes they're generally
characterized by the presence of
polarized hydrogen atoms they can lose a
proton from an O H
so these would be organic acids methanol
can lose this H so can acetic acid they
can also lose a proton from a CH
and this is usually next to a co bond
so this one here would be an H next to a
C double bond o so these qualify as
organic acids the pKa of each of these
methanol is fifteen point five four four
point seven six and this is 19.3 an
organic base has an atom with a lone
pair of electrons that can bond to H+
nitrogen-containing compounds are the
most common organic basis
you
oxygen containing compounds can react as
bases when with a strong acid or as
acids with strong pieces so methylamine
here has this lone pair it's a nitrogen
that's going to act as a base the oxygen
here can act as a base as well and so
can this the oxygen lone pair of acetone
you
so Lewis acids are different from
bronsted acids loose acids are electron
pair acceptor x' so we're dealing with
electrons here and not protons and the
Lewis base is an electron pair donor
bronsted acids are not lewis acids
because they can't accept an electron
directly only a proton would be a Lewis
acid and there's no scale of strengths
as an operon state definition of a PKA
Lewis bases can donate their electrons
Lewis acids can accept electrons if we
look at boron trifluoride which is a
Lewis acid this is the electron space
fill of this here the boron can accept
electron lone pairs from the oxygen here
so this would be the Lewis base this is
the Lewis acid this is the acid base
complex so Lewis bases can accept
protons as well as Lewis acids
most oxygen and nitrogen compounds are
Lewis spaces because they have lone
pairs of electrons
some asses can act as both depending on
the reaction so we can see here some
examples of lewis bases alcohols ethers
aldehydes these all have the electron
lone pairs the sulfur as well nitrogen's
and then we can see phosphorus or this
phosphate compound because of the oxygen
lone pairs so if we look at this which
of these are likely to act as lewis
bases we saw on the last slide oxygen
nitrogen are very good at acting as
lewis bases because of the lone pairs so
this would be and this would be these
two are going to be considered the risk
acids so if we look at an application of
organic chemistry all foods are organic
organic in the dietary industry or in
the food industry is commonly thought to
mean it has no synthetic chemicals or
pesticides or is less dangerous or risky
this is not always a good definition of
that because organic food farmers do use
chemicals and pesticides they might be
natural ones but they do use some to
some extent and the other thing is that
the FDA does not really regulate what is
considered organic or not if there's a
compound that is organic in the whole
food item that can be considered organic
so they don't really police that if
you're making cereal and one ingredient
on there was organic they could slap the
organic label on there so pesticides are
used for weeds they use herbicides for
that for insects
they use insecticides and they use
fungicides for molds and fungi if you
use no pesticides whatsoever and you may
have noticed this if you've ever had a
garden over the summer there is a
significant drop in crop production
which would increase your food costs if
you were a farmer so it's not really
ideal for an organic food farmer to not
use any pesticides at all because they
have to deal with weeds insects or molds
so one of the things that is used is
atrazine which is used on crops there's
100 million pounds used each year to
kill weeds in sorghum corn and sugarcane
and there's trace amounts of this that
persists in the environment exposure can
pose health risks to humans and animals
but there's an alternative but to use
this the EPA will not ban this pesticide
due to the resulting dramatic increase
in food costs currently the benefits
outweigh the risk so what I mean by that
they use atrazine on food crops and if
you use them your crops of sorghum will
increase by 13 bushels increases seven
bushels of corn per acre by not using
these things your food costs and your
farming costs parts a lot more if we
look at the risk versus the benefit
analysis strychnine is a nasty poison
that has a lethal dose of point zero
zero five grams per kilogram which means
that in a group of a hundred people 50
people would die at this dosage level
for arsenic trioxide it would be point
zero one five and you can see the
different things here ethyl alcohol
which is in our alcoholic beverages is
ten point six so they tested the
atrazine in animals at a much higher
level than you would see at normally
exposure the results were converted to
an ld50 value which is at one hundred
people fifty died oh so the results were
converted to an ld50 of value which came
out to be one to four grams per kilogram
that was lethal for 50 percent of the
testing done in animals this has no
information on long-term exposure risk
so there was a small amount of the ld50
is low especially considering that
aspirin also has a similar ld50 so
because of the benefits of the atrazine
outweigh the risk of health the
FDA will continue to allow the use of
atrazine on crops
you
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